Oil burner for low heating capacities

ABSTRACT

An oil burner comprises an oil conveying device, an air blower and a burner head, the burner head including a substantially cylindrical burner tube having an outlet orifice at one end, the burner tube tapering towards the orifice, a burner nozzle situated in the burner tube and having a plurality of supporting arms, and a baffle plate attached to and supported by the supporting arms of the burner nozzle. The burner nozzle is tapered towards an end of the outlet orifice of the burner tube to form a frustoconical end zone at the front end thereof. The baffle plate is of a cup shape and is arranged in the burner tube in front of the burner nozzle with the interior thereof pointing to the outlet orifice of the burner tube. The baffle plate includes a bottom zone conically widening towards the outlet orifice of the burner nozzle and having a central opening with a diameter equal to or larger than that of the burner nozzle, and an edge shell around the bottom zone widening towards the outlet orifice of the burner tube so that an annular gap is defined between a front end of the edge shell of the baffle plate and the inner face of the burner tube.

The invention relates to an oil burner for low heating capacities,particularly for heating capacities ranging from approximately 12 to 30Mcal/h, and comprising an oil conveying device, an air blower and aburner head which comprises a substantially cylindrical burner tube witha section tapering towards its outlet orifice and a burner nozzlearranged in the burner tube as well as a baffle plate which is providedwith a central opening and which is arranged in the burner tube so thatits outer edge is situated in the area of the tapering section and sothat an annular clearance exists between the baffle plate and the innersurface of the burner tube.

Such burners for low heating capacities present considerable problemswith respect to attaining an adequate combustion quality and, bound uptherewith, a good utilisation of the fuel fed to the oil burner and acorrespondingly high efficiency.

Until a solution was found by CH patent specification No. 575 573, ithad to be accepted in oil heating engineering that the combustionefficiency of low heating capacity oil burners is substantially lowerthan that of burners having a relatively high heating capacity. Thecarbon dioxide proportion of the waste gases serves as a measure for thecombustion efficiency. The conditions of oil burners for higher or evennormal heating capacities, namely heating capacities above 50 Mcal/h,cannot be transferred to oil burners for low heating capacities. Infact, it has turned out that on prior art burner heads for oil burnersof a heating capacity below 50 Mcal/h the diameter at the mouth end ismuch too large in relation to the amount of air required for combustionand that the opening in the baffle plate is also proportionally muchlarger than it is on oil burners having a high heating capacity. The lowcombustion efficiency is primarily caused by the fact that the pressurein the burner tube, and accordingly also the flow rate of the air in themixing zone, is substantially less in smaller oil burners than inrelatively large oil burners. A dimensioning of the air blower in asmall oil burner to the effect that the air flows through the opening inthe baffle plate at the same flow rate as in a large oil burner wouldcause an amount of air to be supplied exceeding by far the air requiredfor combustion. This would result in the flame temperature droppingseverely and the carbon dioxide proportion in the exhaust gas becomingvery small and a high proportion of the generated heat being carriedaway with the excessive air in an undesired manner.

On the other hand, throttling the air supply to the value for anapproximately stoichiometric oxygen/fuel mixture would give an air flowhaving only a low flow rate. However, since the mixing effect of theburner head depends to a very large extent on the flow rate of the air,this would result in the air and the fuel being mixed incompletely. Thisin turn would cause the fuel to be only partly burnt, which would resultin a poor efficiency, on the one hand, and a considerable sootformation, on the other hand. Another disadvantage of a large baffleplate opening and a low flow rate is that the pressure differencebetween the interior of the burner tube and the combustion chamberbecomes very small. The relatively large opening in the baffle plate andthe low positive pressure in the burner tube results in the air flow,and thus the combustion efficiency, being very largely influenced bychanges in the pressure in the combustion chamber, such as are usuallycaused, for example, by a change in the chimney draught or by blasts ofwind.

In order to provide an effective remedy for this situation and to obtaina combustion efficiency which is as good as that of conventional oilburners for high and medium heating capacities, the central opening ofthe baffle plate of a generic oil burner has been given a diameter of 11to 14 mm, according to CH patent specification No. 575 573, and the airblower has been dimensioned so that it produces a positive pressuregiving a water column of 32 to 36 mm. In this connection, it was alreadyrealised that for achieving an adequate combustion efficiency in oilburners for low heating capacities it is important to control thepressure ratios in the burner tube through suitable throttling of theamount of overflow of the combustion air fed by the air blower from theburner tube interior into the combustion chamber in the form of primarycombustion air and secondary combustion air. For this purpose, thediameter of the central opening of the baffle plate has been limited inrelation to prior art constructions to values ranging from 11 to 14 mmand the annular clearance between the burner tube outlet orifice and theouter edge of the baffle plate, which clearance serves as the passagefor the secondary combustion air, has also been kept at a value that islow as compared to prior art constructions, so as to allow in thismanner the available flow cross section for primary and secondarycombustion air altogether to be kept adequately small.

This known solution for low heating capacity oil burners givessatisfactory results, but this applies only to oil burners with heatingcapacities above 18 Mcal/h. This known construction has proved to beunsatisfactory for heating capacities which are lower than this becausethe combustion quality as well as the efficiency drop rapidly, as trialshave shown. It has therefore been assumed so far that a heating capacityof 18 Mcal/h is the lower limit for controlling an oil burner of ageneric type so that there is reliably ensured an adequate combustionefficiency along with the operational, economic and ecologicaladvantages bound up therewith.

The task underlying the invention is, while avoiding the describeddisadvantage, to provide with simple and low-cost means a possibility ofreliably controlling the combustion efficiency in oil burners forheating capacities lower than 18 Mcal/h so that it is at least equal tothe known conditions according to CH patent specification No. 575 753.

According to a first principal inventive conception directed at an oilburner for low heating capacities, this is achieved by the invention ina surprisingly simple manner by the combination of various measurestaken in connection with an oil burner of generic type in that, on theone hand, the central baffle plate opening is given a minimum dimensionof 10 to 15 mm and the baffle plate is arranged at a distance of itscentral opening from the free end face of the nozzle body of the burnernozzle which is in the region of between 3 mm at the nozzle connectionend from this free end face and 5 mm at the burner tube outlet orificeend therefrom and in that, furthermore, the flow of primary combustionair passing through the central baffle plate opening is guided in such away that at least in an edge area of the surface of this baffle plateopening this flow flows therethrough approximately vertically to thissurface.

The invention starts out from the realisation, on which it is based asthe general inventive conception, that for extremely low heatingcapacities, namely such of up to twelve Mcal/h it is not sufficient toexert an influence on the air passage cross section between the burnertube interior and the fire chamber in order to solve the problem posedbut that in such low heating capacity ranges other constructionalparameters, to which the experts did not until now attach any importancewhatsoever, gain considerably in significance. On the one hand, thisrelates to the necessity of a further reduction of the lower limit forthe minimum dimension of the central baffle plate opening; whilekeeping, however, a distance of the inlet plane of this baffle plateopening from the free end face of the nozzle body of the burner nozzle.Surprisingly, it was found that this distance does not have to be onlypositive, as the experts deemed necessary until now, the baffle platebeing arranged at the burner tube orifice end of the free end face ofthe nozzle body of the burner nozzle, but that for very specificgeometrical conditions there may be expedient, in the approach flow areaof the primary combustion air to this baffle plate opening, a so-called"negative distance", where the inlet plane of the central baffle plateopening is situated at the nozzle connection end of the free end face ofthe nozzle body of the burner nozzle. A very essential parameter forattaining the desired combustion efficiency at heating capacities whichare lower than the values considered just about manageable until now isthe supply of the primary combustion air to the central baffle plateopening in the direction that is specific to the invention. In thisrespect, the invention is based on the realisation that, particularlyfor such low heating capacities, a thorough mixing of the combustionreactants oxygen and oil mist is essential and that the quality of thismixing process largely depends, particularly in view of the desired highair flow rates, on the fact that it occurs to the largest possibleextent in the starting area of the atomisation cone of the oil mist, inother words as close as possible to the emergence thereof from the freeend face of the nozzle body of the burner nozzle, and that it isdisturbed to the smallest possible extent by turbulences in the area ofthe cone surface. The further development of this inventive conception,which is not obvious, led to the provision according to the inventionthat the primary combustion air passing through the central baffle plateopening is fed to the atomisation cone in the flattest possible mannerover as large a surface area of this baffle plate opening as possiblebut at least over an edge area thereof, that is to say that its flowdirection is approximately axially parallel to the burner tube at leastin this edge area. Surprisingly, this measure brings about aparticularly thorough mixing of the oil droplets of the atomisation conenot only in the envelope area thereof but also in the interior thereof.

It has turned out that due to the invention it is possible to attain foroil burners of the described kind, which are designed for heatingcapacities of as low as 12 Mcal/h, combustion ratios which are at leastas good as those described in CH patent specification No. 575 573 forheating capacities ranging from 18 to 36 Mcal/h. The application of theinvention is not confined to oil burners for heating capacities below 18Mcal/h; on the contrary, the invention can also be advantageously usedfor higher heating capacities, for example for capacities ranging from18 to 35 Mcal/h or even higher capacities.

It has surprisingly turned out that when such low heating capacities areinvolved, it is possible, in the same way as when higher heatingcapacities are involved, to provide a burner nozzle which is arranged inthe burner tube so as to be coaxial therewith and to give the centralopening of the baffle plate a circular shape and a diameter of 10 to 15mm. However, on the other hand, for specific applications it may beexpedient if, according to another inventive conception whichexpediently develops the invention, there are provided two burnernozzles which are symmetrical about the burner tube axis and the centralopening of the baffle plate has the shape of an oval with semi-circularside areas which are coaxial with the centres of the burner nozzles andhave a diameter of 10 to 15 mm. This construction allows a far-reachingadaptation to a relatively wide heating capacity range, particularly ifthe two burner nozzles are each conceived for a minimal oil throughput.

For the rest, a construction of the invention in which the minimumdimension of the central baffle plate opening ranges is known `per se`manner from 12 to 13 mm has proved to be preferable for a large part ofapplications. In this connection, it is pointed out that in the event ofthe central baffle plate opening being circular, the minimum dimensionthereof equals its diameter, whereas if the central baffle plate openingis formed as an oval, this minimum dimension is the vertical distancebetween the two longitudinal sides of the oval.

Particularly favourable flow conditions for the primary combustion airpassing through the central baffle plate opening, along with acorresponding improvement of the combustion efficiency, can be providedin a constructional form of the invention wherein the baffle plate isarranged, at the burner tube orifice end of the nozzle body of theburner nozzle or burner nozzles at a distance of its central opening of1,2 mm from the plane of the free nozzle body end face(s). It issurprisingly that this short distance between the inlet surface of thecentral baffle plate opening and the free end face of the burner nozzleshould have such a specially favourable influence on the combustionefficiency when low heating capacities are involved. As regards thegeometry of the introduction of the oil mist into the fire chamber, thisdistance could actually be further reduced, but it has turned out thatthe space for the accommodation of one or several ignition electrodes isthen so limited that the danger of spark-overs towards the baffle plateor the nozzle body arises.

In a further development of the invention, a preferred constructionalform is characterised, according to another subsidiary inventiveconception, by the guidance of the primary combustion air flow passingthrough the central baffle plate opening by means of the contour of theouter surface of the nozzle body of the respective burner nozzle. In afurther progressive development, the nozzle body of the burner nozzlemay have a contour which tapers remote from the nozzle connection andwhose tangent forms, in the transition area to the free end face of thenozzle body, with the burner tube axis an angle α which is between 15°and 35°, and in this connection it has turned out to be preferable ifthe angle of this tangent in the transition area to the free end face ofthe nozzle body is between 20° and 30°. It has surprisingly been shownthat, provided that the measures of this invention conception areensured, the burner nozzle may well have in known `per se` manner anapproximately frusto-spherical end area at the fire tube orifice end.Until now, it was considered impossible to use for purposes according tothe invention burner nozzle having such a known contour of the outersurface of their nozzle body because the conventional constructions ofsuch burner nozzles usually bring about a guidance of the primarycombustion air flow wherein the flow lines of this combustion air extendat a relatively very steep angle to the element of the oil mist cone,thus courting the danger of pocket of turbulences arising closely behindthe free end face of the nozzle body, on the one hand, and penetratinginto the oil mist cone in a manner that is not very tangential, on theother hand, and therefore bringing about a relatively poor mixing effectof this primary combustion air or its oxygen content with the oil mostdroplet continuum. According to the invention, it will therefore bepreferable to use nozzles of such known design either with an adequatelysmall radius of the frusto-spherical end area of their nozzle body ornozzles in which, due to a relatively small axial dimension of this endarea, the tangent in the transition area to the free end face of thenozzle body extends at an inclination which is within the angle rangeprovided according to the invention. However, these difficulties arevery largely avoided if, according to an inventive alternative, theburner nozzle has in known `per se` manner a frusto-conical end area atthe burner tube orifice end. In this preferred constructional form, itis possible, virtually without any manufacturing, constructional orfitting expenditure, by way of a corresponding inclination of theelement of the cone of the burner nozzle body end area at the burnertube orifice end, to give the tangent in the transition area to the freeend face, which practically coincides with the element of the cone ofthis end area of the nozzle body, the desired inclination which ensuresthat the flow of primary combustion air is always conducted in such away that this air impinges virtually vertically on the inlet area of thecentral baffle plate opening.

It has been found that an oil burner construction according to thisinventive conception and its afore-described advantageous furtherdevelopments leads to a higher pressure drop and, as desired, to acorrespondingly higher speed of the combustion air than is the case inconventional constructions, where the known burner nozzle constructionsnecessitate a larger diameter of the baffle plate hole and the outsidediameter of the baffle plate as well as the burner tube openingtherefore also have to be made larger since otherwise the flame contactsurface would become too small, which would inevitably lead to anenlarged cross-sectional opening at the burner head end with the resultthat in such a conventional construction it is impossible to attain thedesired high air pressure in the burner tube. The increased combustionair speed achievable, in contrast thereto, due to the burner headconception according to this inventive idea and its advantageous furtherdevelopments, also substantially contributes to promote the mixing ofcombustion air an oil mist in that, surprisingly, a short-term andthorough intermixing of the oil mist and oxygen is brought about and acomplete, almost stoichiometric combustion is ensured.

However, another further development of this inventive conception, whichis not obvious, has likewise had proven success, and in this the freeend face of the nozzle body of the burner nozzle has a diametercorresponding at the most to half of the smallest dimension of thecentral baffle plate opening or the diameter thereof. Experience hasshown that the diameter of this free end face of the nozzle body shouldbe approximately 5 mm. Experience has shown that this measure results inparticularly good aerodynamic conditions in the burner head with a smallcross-sectional opening at the burner head end, as desired, and acorrespondingly high air compression in the burner tube along with acomparatively higher combustion air speed, and in this connection it hasturned out that the very short sojourn time of the oil mist until itscombustion can be utilised at an increased proportion for as rapid andthorough intermixing of the oil mist with the combustion air aspossible, since the combustion air can act on this mixing processalready in the area of the peak of the oil mist atomisation cone.

It has furthermore proved to be advantageous if the burner nozzle has,in known `per se` manner, a hexagonal area at the nozzle connection endand if, in a further development of the last-mentioned inventiveconception, the enveloping circle diameter thereof is smaller than theminimum dimension or the diameter of the central baffle plate opening.This, too, causes the aerodynamic flow geometry of the primarycombustion air to be considerably promoted on the lines of attainingstoichiometric combustion conditions.

The invention is actually not confined to the inner design of the burnernozzle specifically used, particularly in view of the angle of cone ofthe oil mist atomisation cone produced by it, but it has surprisinglyturned out that known `per se` burner nozzles providing an oil mistoutflow cone with an included angle ranging from 60° to 70° may well beused, this included angle preferably being 65°.

The measure according to another subsidiary inventive conception hasprovided an entirely surprising result in that it has turned out that,particularly for heating capacities below those considered until now tobe just about still manageable, namely heating capacities below 18Mcal/h, an increase in the combustion efficiency can be brought about inthat the possibility of secondary combustion air flowing into the firechamber is restricted even further in relation to the solution knownfrom CH patent specification No. 575 573. To this end, this furtherinventive conception makes provision for two measures which can be usedalternatively or jointly and one of which is characterised in that thereis provided an annular clearance opening for the passage of secondarycombustion air between the outlet orifice of the burner tube and theouter edge of the baffle plate, the width of which opening is at themost 0.5 mm in the plane of the burner tube orifice, while according tothe other measure there is provided an annular clearance opening for thepassage of secondary combustion air between the outlet orifice of theburner tube and the outer edge of the baffle plate, the depth of whichopening is at the most 1.3 mm in the direction of the burner tube axis.

It has furthermore been shown that the burner conception according tothe invention also allows the outer edge diameter of the baffle plate torange, in known `per se` manner, from 40 to 70 mm. This means thatexisting oil burners can be converted in an extremely economic manner tolower heating capacities giving an excellent degree of combustion and ahigh efficiency according to the invention in that all that is needed isto use a baffle plate which is suitable for the purposes according tothe invention and a burner nozzle imparting to the primary combustionair a flow according to the invention in a co-ordination that isspecific to the invention. The same applies, of course, to the use ofburner nozzles in a tandem arrangement side by side. This brings aboutnot only easier assembly for the first equipment and for conversions butalso a rationalisation of the entire manufacturing and deliveryprogramme as regards production and storage along with the correspondingcost advantages. It has proved to be particularly preferable for theouter edge diameter of the baffle plate to range from 49 to 55 mm and tobe preferably 50 mm.

Similarly, the diameter of the outlet orifice of the burner tube mayrange from 41 to 71 mm, the preferred range being between 50 and 56 mmand this diameter preferably being 51 mm.

In a further development of the invention, the baffle plate may be ofcup-shaped design and may be arranged within the burner tube, itsinterior pointing to the outlet orifice of the burner tube. The baffleplate may have in known `per se` manner a bottom section which extendsapproximately in the plane of its central opening and continues in ashell expanding conically towards the burner tube outlet orifice.However, improved combustion results, particularly with a view toimproving the flame conduction at the combustion chamber end of thebaffle plate, can surprisingly be brought about by another preferredconstructional form of the invention, wherein the baffle plate has abottom area which expands conically towards the outlet orifice of theburner tube and an edge shell which, with a more pronounced inclination,also expands towards the outlet orifice of the burner tube. It ispreferred if the angle of inclination of the conically expanding bottomarea of the baffle plate towards the surface of the central openingthereof ranges from 15° to 30° and preferably at 25°.

Whereas in the known baffle plate of cup-shaped design and having a flatbottom area there are provided 12 passage slots for the primarycombustion air, which slots are arranged in this bottom area, provisionmay be made, according to an expedient further development of thisinventive conception, particularly if a baffle plate of cup-shapeddesign is used which has a conically expanding bottom area, in theinterests of a stable conduction of the flame in conjunction with anintensification of the intermixing of the primary combustion air withthe oil mist continuum, particularly in the jet cone zone, so as toincrease the combustion efficiency, for the central opening of thebaffle plate to widen into a number of three to twelve, preferably fourpassage slots for the primary combustion air, which slots are recessedin a bottom area of the plate. These passage slots may open in known`per se` manner into the central baffle plate opening either radially orinclined at an angle to the radial direction. They may furthermore bearranged in known manner so that they are inclined through 30° to 60°towards the longitudinal axis of the burner tube. Particularly if theyare used in conjunction with a conically widening bottom area of thebaffle plate, advantages in the conduction of the combustion can beattained, particularly for specially low heating capacities.

In a further inventive development, the passage slots may be openingswhich are formed by approximately strip-like zones of the baffle platebottom area being bent in a flag-like manner from the plane of thebottom area in the direction of the outlet orifice of the burner tube.This novel baffle plate conception is not only expedient for achieving aparticularly good combustion efficiency at extremely low heatingcapacities but also provides the advantage of an easy and economicproduction of the baffle plate. In this connection, it has proved to bepreferable if the strip-like zones are bent from the plane of the baffleplate bottom area without being buckled. This avoids shock-freedeflections of the primary combustion air flows passing through theslots, which may lead to the formation of turbulences and thus toimpairments of the mixing efficiency of the primary combustion air andthe oil mist in the envelope area of the jet cone. Furthermore, it hasproved to be particularly preferable if the baffle plate areas boundingan opening forming a passage slot overlap one another. A value ofbetween 0.2 and 1 mm has been established for the width of the passageslots, and in this connection it has turned out to be preferable if thewidth of the passage slots ranges from 0.4 to 0.5 mm. In this way,primary combustion air is fed at the fire chamber end from the baffleplate to the jet cone envelope of the oil mist at a speed that isadequate for thorough intermixing, on the one hand, and, on the otherhand, it is ensured that a substantial proportion of the primarycombustion air passes through the central baffle plate openingapproximately in the axial direction and, in a zone of the jet cone thatis particularly close to the starting area thereof, acts on this cone ata particularly flat angle of application relative to this cone and thusutilises its own speed components for pulling apart the oil droplets inthe jet cone substantially in the direction of the main component oftheir speed and thus to contribute to a particularly thoroughintermixing of the oil mist with primary combustion air.

According to another principal inventive conception, the invention alsorelates to a method for the operation of the oil burner according to theinvention with light or extra light heating oil. This method ischaracterised in that a constant positive air pressure of a water columnof 32 to 36 mm is maintained in the interior of the burner tube and inthat the adaptation of the air supply to the oil amount to be conveyedis effected by the displacement of the baffle plate in such a way thatan excess of air of a maximum of 5% by volume is provided, whichcorresponds to a carbon dioxide proportion in the exhaust gases of atleast 15% by volume. This conduction of the method is known from CHpatent specification No. 575 573 for oil burners for heating capacitiesfrom 18 Mcal/h onwards. Surprisingly, it was found by the invention thatthis method is not only possible in the case of lower heating capacitiesof down to approximately 12 Mcal/h but also provides the optimumconditions for the conduction of the combustion process for thisextremely low heating capacity range. These advantages come to lightparticularly clearly in a burner construction according to the inventionproviding a frusto-conical end area of the nozzle body of the burnernozzle and a baffle plate having a conically widening bottom area,because due to the application of this known `per se` conduction of themethod another advantageous effect is surprisingly achieved on an oilburner according to the invention designed in this way, which consistsin that despite the small geometrical dimensions in the area between theburner nozzle and the baffle plate and particularly the central openingthereof, there arise, due to the air compression in the burner tubewhich is relatively high for these small dimensions, flow rates of theprimary combustion air in the area between the burner nozzle and thebaffle plate which allow an expedient arrangement of the ignitionelectrodes at an extremely small distance from one another and from theburner nozzle body, on the one hand, as well as of the baffle plate, onthe other hand, and simultaneously ensure that the ignition spark isdirected into the flow direction and sparks are reliably prevented inthis way from jumping on the nozzle body of the burner nozzle or thebaffle plate. The advantageous consequence is not only a specialuniformity of combustion with a corresponding increase in the combustionefficiency resulting in the advantageous consequences thereof inrelation to the operating time, as already described, but also animprovement of the stability of the components of the burner head due toa reduction of the scaling corrosion thereof and thus, in the finalanalysis, a considerable increase in the total economy of such an oilburner.

Hereinafter, the invention will be described in more detail, purely byway of example, with reference to two preferred exemplified embodiments,which are shown in the drawing, and this also with respect to furtherfeatures and advantages brought about by these. In the drawing:

FIG. 1 shows a longitudinal section through a burner head,

FIG. 2 shows a view of the baffle plate in the direction of visiondesignated II in FIG. 1,

FIG. 3 shows, in a representation corresponding to that of FIG. 2,another constructional form of a baffle plate which is intended for usewith two burner nozzles in a tandem arrangement, and

FIG. 4 an enlarged detail section of the baffle plate along the lineIV--IV shown in FIG. 2,

The burner head shown in FIG. 1 and designated 3 in its entiretycomprises a burner tube 4 with a section 4b that tapers conically as faras its outlet orifice 4a and a burner nozzle 5 which is coaxiallyarranged in the burner tube 4 and serves for atomising the oil. Thediameter of the outlet orifice 4a ranges expediently from 41 to 71 mmand is advantageously approximately 50 to 56 mm. A diameter value of 51mm is preferred. The burner nozzle 5, which comprises a burnerconnection 12 and a nozzle body 13, may advantageously be designed likethat described in CH patent specification No. 553 379. The outsidediameter of the burner connection 12 is, for example, 12 mm, but shouldbe as small as ever possible. At the end that is directed towards theburner connection 12, the nozzle body 13 is provided with a hexagonalsection, whose enveloping circuit diameter is smaller than the diameterof a central opening 6a of a baffle plate 6. Expediently, it may besmaller than the diameter of the burner connection 12. At the other end,the nozzle body 13 has a conically tapering section 13a, the diameter ofthe taper at the free front end, i.e. in the plane of the nozzle mouth,being at the most half of the diameter of the central baffle plateopening 6a and preferably 5 mm. The round baffle plate 6 arrangedforward of the mouth of the burner nozzle 5 in the burner tube 4 iscup-shaped in design and is so arranged in the burner tube 4 that itopens in the direction that is opposite to the burner nozzle 5. Itcomprises a bottom area 6c, which conically widens away from the burnernozzle 5 at an angle of inclination β relative to the verticalcross-sectional plane, which ranges from 15° to 30° and is preferably25°, and has the central circular opening 6a, and an edge shell 6e whichadjoins the latter towards the outside and conically widens at a morepronounced inclination in the same direction, i.e. towards the burnertube outlet orifice 4a and whose end forms the outer edge 6b of a baffleplate 6. As emerges from FIGS. 2 and 3, the bottom area 6c is providedwith a number of passage slots 6d ranging from three to twelve--four inthe exemplified embodiment shown--which extend from the opening 6a toclose to the edge of the bottom area 6c. The passage slots 6d extendradially or at an angle to the radial direction and are inclined towardsthe direction of the longitudinal axis 14 of the burner tube 4 throughapproximately 30° to 60°. The diameter of the baffle plate opening 6amay be approximately 10 to 15 mm, however preferably 12 to 13 mm. Themaximum outside diameter of the nozzle body 13 is advantageously chosento be of the same size as or somewhat smaller than the diameter of thebaffle plate opening 6a.

The end area 13a of the nozzle body 13, which tapers conically to adiameter of 5 mm of the free end face 13b, has a conicity whosedimension is given by the angle α which the tangent 17 in the transitionzone to the free end face 13b of the nozzle body 13 forms with theburner tube axis 14. In the exemplified embodiment shown, this tangent17 coincides with the generatrix of the conically tapering area 13a ofthe nozzle body 13. If a burner nozzle having a known `per se`frusto-spherical end area is used, then it must be ensured that thecontour thereof is such that the tangent thereon in the transition zoneto the free end face also lies within an angle range provided accordingto the invention. This angle range for the inclination of the tangent 17in the transition zone to the free end face 13b with the burner tubeaxis 14 or a parallel thereto is between 15° and 35°, tangent angles αranging from 20° to 30° being preferred.

The baffle plate 6 is so arranged that its central opening 6a keeps anegative distance of up to 3 mm or a positive distance of up to 5 mmfrom the free nozzle body end face 13b. A "negative distance" is adistance which is provided when the baffle plate opening 6a takes up aposition at the nozzle connection end of the free end face 13b of thenozzle body 13, while a "positive distance" is provided when this baffleplate opening 6a is located at the burner tube orifice end of the nozzlebody 13. Particularly preferred is an arrangement of the baffle plate 6at the burner tube orifice end at a distance a of its central opening 6afrom the free nozzle body end face 13b of 1.2 mm.

On the other hand, the baffle plate 6 is so arranged that its outer edge6b is situated in the area of the tapering burner tube section 4b andthat there is left between its outer edge 6b and the inner surface 4c ofthe burner tube 4 an annular gap 7, whose width b in the plane of theburner tube orifice 4a is at the most 0.5 mm and whose depth t in thedirection of the burner tube axis 14 is at the most 1.3 mm. The outeredge diameter of the baffle plate 6 ranges from 40 to 70 mm and shouldpreferably range from 49 to 55 mm. A baffle plate having an outer edgediameter of 50 mm is preferred. For the rest, the baffle plate 6 is heldin such a way that it can be moved along the burner tube axis 14. Thisallows the cross-sectional space in the annular gap 7, which space is tobe made available for the through-flow of secondary combustion air, tobe varied within the indicated limits when the baffle plate 6 isdisplaced, so that the desired air compression can be adjusted in thismanner in the burner tube 4. To this end, the baffle plate 6 isconnected to the burner nozzle 5 by a holder which has three supportingarms 9 fastened to a sleeve 10. The sleeve 10 is displaceable on theburner connection 12, which may be provided with a graduation, and maybe clamped with the aid of a screw. The supporting arms 9 are eachprovided with a strap 9a which extends approximately radially to theoutside and rests against the inner face 4c of the cylindrical burnertube section 4d. By this means, the burner nozzle 5 and the baffle plate6 are centred without substantially disturbing the air flow in theburner tube. The burner connection 12 is carried in a flange at theleft-hand end of the burner tube 4, which is not shown in FIG. 1, sothat it can be displaced together with the baffle plate 6 in thedirection of the longitudinal axis 14. In order to facilitate thecorrect setting of the baffle plate 6, there may be arranged on theoutside of the flange a setting device, for example a micrometer screw.In the burner head 3, there is furthermore provided an ignitionelectrode 15 which is indicated diagrammatically in FIG. 1.

The burner connection 12 end that is remote from the nozzle body 13 isconnected to an oil conveying device 1 by a line and the interior 8 ofthe burner tube 4 is connected to an air blower 2. All the air suppliedby the air blower 2 is fed directly to the interior 8 of the burner tube4 so that neither the connecting line nor the burner tube needs to havean air flap.

For the operation of the oil burner according to the invention, which isconceived for a heating capacity ranging preferably from approximately12 to 30 Mcal/h but can also be used, without difficulty, for higherheating capacities, light or extra light fuel oil, which has, as isknown, a heating value of the order of approximately 10 Mcal/kg, is fedto the burner nozzle 5 by the oil conveying device 1. Furthermore,combustion air is fed to the burner tube by the air blower 2, the airblower 2 being so dimensioned that a positive pressure giving a watercolumn of 32 to 36 mm is formed in the interior 8 of the burner tube 4.A displacement of the baffe plate 6 now causes the annular gap 7 to beset in such a way that there is fed to the flame an amount of air whichprovides an approximately stoichiometric air/fuel mixture or at the mostan excess of air of approximately 5% by volume. This can be detected,for example, by measuring the carbon monoxide and carbon dioxideproportion of the exhaust gases. For the operation of the oil burner,there may then be fitted an appropriate scale on the setting deviceserving for displacing the burner nozzle and the baffle plate.Surprisingly, the pressure in the burner tube 4 remains constant withinthe limits indicated above even for oil burner heating capacities belowthe minimum application value of the oil burner according to CH patentspecification No. 575 573 of 18 Mcal/h. In particular, the essentialadvantage thereof is that the flow rate at which the air flows throughthe baffle plate opening 6a and the annular gap 7 is approximatelyindependent of the amount of oil conveyed, so that the burner headprovides approximately the same mixing effect at every heating capacitywithin the indicated interval. Furthermore, the oil burner conceptionaccording to the invention causes the flow direction of the primarycombustion air flow passing through the baffle plate opening 6a toextend, at least in an edge area of this baffle plate opening,approximately vertically to the area thereof. The oil burner designedaccording to the invention makes it possible, even at heating capacitiesbelow 18 Mcal/h, to attain a carbon dioxide proportion which is higherthan 15% by volume in the entire working zone. In a combustion in whichso much air is supplied that a stoichiometric oxygen/fuel mixture isprovided, the carbon dioxide proportion of the exhaust gases in theusual light and extra light fuel oils is approximately 15.7% by volume.The combustion efficiency of the oil burner designed according to theinvention is thus very close to the maximum value obtainable with astoichiometric mixture even for heating capacities below 18 Mcal/h. Thishas the advantage that a high flame temperature is given and that thelosses caused by the heat transport of the exhaust gas are relativelysmall.

The detachable fastening of the baffle plate 6 to the burner nozzle 5makes it possible to adapt the distance between the baffle plate 6 andthe mouth of the burner nozzle 5 to the atomisation angle of the latter.Once the setting has been effected, the baffle plate 6 is then onlydisplaced together with the burner nozzle 5 or the burner connection 12thereof.

As FIGS. 2 and 4 reveal, the baffle plate 6 has another special featureas compared to conventional baffle plate constructions, in addition toits conically widening bottom area 6c. This is to the effect that thepassage slots 6d in the bottom area 6c are formed by apertures which areproduced in that approximately strip-like areas 18 of the bottom area 6cmaterial are bent in an approximately flag-like and virtuallybuckle-free manner from the plane of this bottom area 6c in thedirection of the interior of the cup-shaped structure formed by thebaffle plate 6, overlapping of the edge zones bounding the slot-shapedapertures 6d being brought about, for example, by slight upsetting inthe circumferential direction. This overlapping is designated v in FIG.4. The width s of the passage slots 6d should range from 0.2 to 1 mm andis preferably 0.4 to 0.5 mm.

Another constructional form of an oil burner according to the inventionfor particularly low heating capacities is diagrammatically shown inFIG. 3. It comprises two burner nozzles which are placed side by side ina tandem arrangement and whose nozzle body contours are particularlysmall, and accordingly the baffle plate 6 is provided with a centralopening 6a which has the shape of an oval, whose lateral edge zones areformed as semi-circular arcs whose diameter, which corresponds to thesmallest dimension of the baffle plate opening 6a, i.e. to the verticaldistance between the two longitudinal edges thereof, ranges from 10 to15 mm and is preferably 12 to 13 mm. In this construction form of theinvention, virtually the same favourable conditions with respect to theindividual operating parameters are given as in the constructiondescribed first.

We claim:
 1. An oil burner including an oil conveying device, an airblower and a burner head, said burner head comprising:a substantiallycylindrical burner tube having an outlet orifice at one end, said burnertube tapering towards the orifice, a burner nozzle situated in saidburner tube, said burner nozzle having a plurality of supporting armsand being tapered towards said outlet orifice of said burner tubethereby to have a frustoconical end zone at a front end thereof, and abaffle plate attached to and supported by the supporting arms of saidburner nozzle, said baffle plate having a cup shape and being arrangedin said burner tube in front of said burner nozzle with the interiorbottom of the cup shape facing the outlet orifice of said burner tube,the cup shape of said baffle plate including a bottom zone conicallywidening towards the outlet orifice of said burner tube and having acentral opening with a diameter equal to or larger than that of theburner nozzle and a plurality of passage slots radially outwardlyextending from the central opening, said passage slots being recessed bybends in the bottom zone forming strip zones extending angularly towardsthe outlet orifice of said burner tube, and an edge shell around thebottom zone widening towards the outlet orifice of said burner tube sothat an annular gap is defined between a front end of the edge shell ofsaid baffle plate and the inner face of said burner tube, wherebyprimary air in the burner head is conducted over said frustoconical endzone of said burner nozzle mainly into the central opening of saidbaffle plate to flow substantially parallel to an axis of said burnertube in at least the edge zone of said central opening of said baffleplate for imparting thorough mixing of air and oil, and secondary air inthe burner head is ejected through said annular gap for completecombustion of oil.
 2. An oil burner according to claim 1, in which saidpassage slots are overlapped by the respective strip zones.
 3. An oilburner according to claim 2, in which three to twelve passage slots areprovided around the central opening of the bottom zone of the baffleplate.
 4. An oil burner according to claim 3, in which said passageslots are inclined relative to the longitudinal axis of the burner tubefrom 30° to 60°.
 5. An oil burner according to claim 4, in which thewidth of the passage slot is from 0.2 to 1 mm.
 6. An oil burneraccording to claim 5, in which the width of the passage slot is from 0.4to 0.5 mm.
 7. An oil burner for heat capacities from 12 to 30 Mcal/haccording to claim 1, in which said air blower is adapted to produce apositive pressure in the interior of the burner tube of 32 to 36 mmwater, the outer edge diameter of the edge shell of the baffle plate isbetween 49 and 55 mm and the diameter of the central opening in thebottom zone of the baffle plate is between 12 and 13 mm, the front endof the frustoconical end zone of said burner nozzle is situated relativeto said central opening in the bottom zone of the baffle plate between 3mm at the burner nozzle side and 5 mm at the outlet orifice side of theburner tube, and the annular gap between the front end of the edge shellof the baffle plate and the inner face of the burner tube has a width nogreater than 0.5 mm in the plane of the burner tube outlet orifice. 8.An oil burner according to claim 7, in which two burner nozzles areprovided in the burner tube symmetrically situated about an axis of theburner tube, the central opening of the baffle plate having an ovalshape with semi-circular side areas coaxial with the centers of theburner nozzles.
 9. An oil burner according to claim 7, in which thefront end of the frustoconical end zone of said burner nozzle issituated relative to said central opening of the baffle plate at adistance of 1.2 mm at the outlet orifice side of the burner tube.
 10. Anoil burner according to claim 7, in which said frustoconical end zone ofthe burner nozzle has a contour whose tangent relative to the axis ofthe burner tube forms an angle between 15° and 35°.
 11. An oil burneraccording to claim 10, in which said angle of the tangent of the endzone of the burner nozzle relative to the axis of the burner tube isbetween 20° and 30°.
 12. An oil burner according to claim 7, in whichthe front end of said burner nozzle has a diameter no greater than halfof the central opening of the baffle plate.
 13. An oil burner accordingto claim 12, in which the diameter of the front end of the burner nozzleis 5 mm.
 14. An oil burner according to claim 7, in which the front endof the edge shell of the baffle plate is situated in the burner tube ata distance no greater than 1.3 mm from the front end of the burner tube.15. An oil burner according to claim 7, in which the inclination angleof the bottom zone conically widening towards the outlet orifice of theburner tube relative to a vertical line is from 15° to 30°.
 16. An oilburner according to claim 15, in which the inclination angle of thebottom zone is 25°.